Experimental analysis

3.7.4.1 Effect of different reinforcement bars on bond behaviour

At the same concrete strength, the depth of ribs as well as the distance between them plays the main role to determine the relation between bond stress and slip.

0.0 0.5 1.0 1.5 2.0 2.5

0 2 4 6 8 10 12 14 16 18

Slip [mm]

Bond stress [N/mm²] .

ILWC + GFR

ILWC + GFR with head

ILWC + SRFT

Figure 3.22: Bond stress versus slip for different types of reinforcement with ILWC.

In Fig. 3.22, the maximum bond stresses were 0.87 MPa at 0.703 mm in the case of GFR, 1.04 MPa at 0.169 mm in the case of SRFT, and 1.99 MPa at 13.86 mm in the case of GFR with head bolt. In the first two cases, the bond stress depends on the distribution of compression stress on the surrounding matrix, which is better in the case of SRFT because of the large number of ribs per unit length as shown in Figures 3.23a & 3.23b.

a. b. c.

Figure 3.23: Strut-and-tie model for different reinforcement bars a) SRFT; b) GFR; c) GFR with head bolt

In other words, the number of ribs per unit length in the case of SRFT is 25 % more than that in the case of GFR, which increases the maximum bond stress compared to GFR by 20 %.

After the bond stress reaches its maximum level, the bond stress depends on the friction between the outer surface area of ribs and the surrounding matrix which is smaller in the case of SRFT than that in the case of GFR by 53 %, this leads to the suddenly drop in the bond stress in the case of SRFT after the maximum bond stress reaches. Using head bolts at the end of GFR enhances the transition of the force from the bar to surrounding concrete. The bond stress in this case depends mainly on the compression and tensile strength of the concrete (Figure 3.23c). Using the head bolt with GF bars increases the slip at maximum bond stress by about 20 times and increases the maximum bond stress more than double.

3.7.4.2 Effect of polypropylene fibres on bond behaviour

In order to improve the splitting tensile strength of ILWC, PP fibres (1.0 kg/m³) were used with different lengths; 6, 12, and 20 mm, which increased the tensile strength with 10 %, 23 %, and 30 % respectively. The long PP fibres increase the amount of energy being absorbed in the de-bonding of the fibre from the concrete matrix prior to the complete tension failure of concrete. On the other hand, adding PP fibres reduced the compact ability of the cement matrix and in consequence reduced the compressive strength with 56 %, 43 %, and 41 % respectively. Therefore, using of long PP fibres is more suitable than small ones, especially in low strength materials.

0.0 0.2 0.4 0.6 0.8 1.0

0 1 2 3 4 5 6 7

Slip [mm]

Bond stress [N/mm²] .

6 mm PP fibres

12 mm PP fibres 20 mm PP fibres

Without PP fibres ILWC + GFR bar

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0 1 2 3 4 5 6 7

Slip [mm]

Bond stress [N/mm²] .

6 mm PP fibres

12 mm PP fibres 20 mm PP fibres

Without PP fibres

ILWC + SRFT bar

Figure 3.24: Bond stress versus slip for GFR with Figure 3.25: Bond stress versus slip for SRFT with different lengths of PP fibres. different lengths of PP fibres.

Adding the 6 mm PP fibres to ILWC reduced the maximum bond stress in the cases of GFR and SRFT by 37 % and 2.4 % respectively, while adding the 20 mm PP fibres increased the maximum bond stress in the cases of GFR and SRFT by 4.6 % and 25.3 % respectively. In the case of 20 mm PP fibres, the reduction of compressive strength balanced with the increasing of the tensile strength (Figures 3.24 & 3.25). Another important note is that using of PP fibres

reduced the slip at maximum bond stress especially in the case of GFR, which is better for crack width control in serviceability limit state.

0.0 0.5 1.0 1.5 2.0 2.5

0 5 10 15 20 25

Slip [mm]

Bond stress [N/mm²]

ILWC + 6 mm PP fibres ILWC +

12 mm PP fibres

ILWC + 20 mm PP fibres ILWC without

PP fibres

ILWC + GFR with head

Figure 3.26: Bond stress versus slip for GFR with head bolt with different lengths of pp fibres.

As it shown in Figure 3.26, adding PP fibres to the mixture of ILWC has a negative effect on the bond behaviour in the case of GFR with head bolt. In this case, the bond stress depends in on the compressive strength of ILWC (Figure 3.23c), which is reduced due to the loss of workability after adding PP fibres.

3.7.4.3 Effect of confinement on bond behaviour

Another effective method was used to improve the radial tensile strength in the case of GFR with head bolt (Figure 3.23c). The stirrups are used as a confinement tools with different numbers (1, 2, and 3 stirrups with 12 cm side length). Using the stirrups in the cases of SRFT and GFR bars without head bolt has no significant effect on the bond behaviour.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0 10 20 30 40 50 60 70

Slip [mm]

Bond stress [N/mm²]

No Stirrups 1 Stirrups 2 Stirrups 3 Stirrups ILWC + GFR with head

Figure 3.27: Bond stress versus slip for GFR with head bolt using different numbers of stirrups.

The effect of using different confinement ratios on the behaviour of bond in the case of GFR with head bolt is shown in Figure 3.27. The slope of the bond stress-slip curve after the maximum bond stress as well as the number of cracks and crack width are reduced by increasing the confinement ratio. In the case of using 3 stirrups, the first crack starts at slip of 55 mm, while it starts after 12 mm slip in the case of using no stirrups.

The previous experimental analysis for the bond behaviour in ILWC concludes that at the same concrete strength, the configuration and the number of the ribs on the surface of the bars play the main role on the relationship between bond stress and slip. Optimizing the shape of ribs and using additional parts such as the head bolt (Fig. 3.23c) are essential to enhance the bond behaviour through the good transition of the force from the bars to concrete. Using of PP fibres with length not less than 20 mm in ILWC improves the bond behaviour especially with bars that have more numbers of ribs per unit length as in SRFT. Adding PP fibres reduces the slip between the bars and ILWC at the maximum bond stress, which is required to control the crack width in serviceability limit state. Using the confinement stirrups reduces the number and the width of cracks especially in the case of GFR with head bolt, where the ring tensile strength is more effective than in the cases of SRFT and GFR without head bolt. The slope of the bond stress-slip curve after the maximum bond stress is reduced by increasing the confinement ratio (Fig. 3.27).